The Warrior Heir by Cinda Williams Chima
I have read almost every children's fantasy book out there and few surprise me anymore...but this one was FANTASTIC. Cinda Williams Chima is a talented writer with an imagination that details the most absurdly complex parallel world in which magical guilds are undergoing a classic power struggle. It is a fresh take on an old concept.
This book kept me on the edge of my seat. It was a wild ride through betrayals and forgiveness and left me clamoring for more.